Transmission device, communication system, and precoding computation method

ABSTRACT

A transmission device to be applied to a communication system configured to carry out multi-user MIMO downlink communication, includes a precoder configured to apply transmission precoding processing, a nonlinear processor configured to carry out successive interference cancellation, a beam forming controller configured to form a plurality of beams and to control a beam radiation direction, and a user ordering processor configured to estimate a positional relationship among the plurality of user terminals from radiation direction information on each of the beams, order users in ascending order of a distance from a specific user, and determine a pairing of a transmission target user and an interference permissible user. The precoder determines two users of the transmission target user and the interference permissible user for each user based on the pairing result, and carries out null steering for users other than the two users, to thereby carry out the transmission precoding processing.

TECHNICAL FIELD

The present invention relates to a transmission device, a communicationsystem, and a precoding computation method, which are to be used inmulti-user MIMO downlink communication.

BACKGROUND ART

In recent years, as a wireless communication system for achieving ahigh-speed transmission in a limited frequency band, there has beenactively studied a multi-user multiple-input multiple-output (MU-MIMO)system obtained by extending a MIMO system, in which a plurality ofantennas are installed both in a receiver and a transmitter, so that thespace division multiple access (SDMA) may be applied to the MIMO system.

The MU-MIMO is a system form in which a plurality of user terminals eachincluding a plurality of antennas exist for a base station including aplurality of antennas, and simultaneous transmission is carried out forthe plurality of terminals in the same wireless frequency band.

A subject of the present invention is an MU MIMO downlink fortransmitting a signal from the base station to the respective userterminals. FIG. 1 is an explanatory diagram for illustrating the MU-MIMOdownlink, which is the subject of the present invention.

In this case, the number of reception user terminals is indicated byN_(usr) (N_(usr)≥2), the number of antennas of a user # u (u=1, 2, . . ., N_(usr)) is indicated by N_(r,u) (N_(r,u)≥1), and the number ofantennas of the transmission base station is indicated by N_(t)(N_(t)≥1). Therefore, the total number of reception antennas of all theusers is given as follows.N _(r,total) =N _(r,1) +N _(r,2) + . . . +N _(r,Nusr)

Moreover, an N_(r,u)×N_(t) channel matrix from the transmission basestation to the user # u is indicated by H_(u). In the MU-MIMO downlink,the signals are simultaneously transmitted from the base station to therespective user terminals. Therefore, reception signals received at theuser terminal generally include signals to be transmitted to other usersas well as a desired signal. In other words, inter-user interference(IUI) occurs.

It is desired that a countermeasure against the IUI be taken as much aspossible at a base station, in which restrictions on a processing amountand the number of the antennas are less strict than those on the userterminal. Thus, in the MU-MIMO downlink, transmission precoding carriedout at the base station is indispensable as the countermeasure againstthe IUI.

As representative precoding for achieving the MU-MIMO downlink, blockdiagonalization (BD) is widely studied (for example, refer to Non PatentLiteratures 1 and 2). The BD method is a precoding method of directingnull to users other than a desired user to form a beam space so that thesignal is transmitted only to the desired user.

An MU-MIMO environment in which the IUI does not occur at all can beachieved by carrying out this operation for all the users, and areceiver configuration in the terminal can consequently be simplified.Meanwhile, the BD method consumes the degree of freedom of the antennasfor the IUI removal (nulling), and has thus a problem in that a greatreduction in desired power occurs.

To address this problem, there is proposed a nonlinear-BMD (NL-BMD)method, which combines a block multi-diagonalization (BMD) method ofpermitting IUI of a part of users to secure a diversity gain and amethod of suppressing residual IUI through pre-canceling with each other(for example, refer to Non Patent Literatures 3 to 5).

CITATION LIST Non Patent Literature

[NPL 1] M. Rim, “Multi-user downlink beamforming with multiple transmitand receive antennas,” Electron. Lett., vol. 38, no. 25, pp. 1725-1726,December 2002.

[NPL 2] L. U. Choi and R. D. Murch, “A transmit preprocessing techniquefor multiuser MIMO systems using a decomposition approach,” IEEE Trans.Wireless Commun., vol. 3, no. 1, pp. 20-24, January 2004.

[NPL 3] Hiroshi Nishimoto, Akinori Taira, Hiroki Iura, Shigeru Uchida,Akihiro Okazaki, and Atsushi Okamura, “A Study on Nonlinear BlockMulti-diagonalization Precoding for High SHF Wide-band Massive MIMO in5G”, IEICE technical report, vol. 115, no. 472, RCS2015-377, pp.255-260, March 2016.

[NPL 4] H. Harashima and H. Miyakawa, “Matched-transmission techniquefor channels with intersymbol interference,” IEEE Trans. Commun., vol.20, August 1972.

[NPL 5] B. M. Hochwald, C. B. Peel, and A. L. Swindlehurst, “Avector-perturbation technique for near-capacity multi-antenna multiusercommunication—Part II: Perturbation,” IEEE Trans. Commun., vol. 53, no.3, pp. 537-544, March 2005.

SUMMARY OF INVENTION Technical Problem

However, the related art has the following problems.

With the linear precoding method represented by the BD method, it isknown that when a user to be excluded, namely, a user to which the IUIpre-cancellation is to be applied, is close in distance, a desired powergreatly decreases, and thus the signal-to-noise power ratio (SNR)deteriorates. The BMD method is also a type of liner precoding, and theSNR thus decreases when a user to be excluded is close in distance.

Meanwhile, the IUI pre-cancellation (IUI PC) through the pre-cancelingis a non-linear operation on the transmission side, and provides astable transmission characteristic almost independently of the positionof a user to which the IUI pre-cancellation is to be applied. However,the arrangement of users cannot be determined in advance, and varies asthe time elapses. Therefore, a close user is not always specified as theIUI permissible user. As a result, there is a problem in that a closeuser is specified as the subject of the IUI pre-cancellation through theBMD method, and the SNR decreases as in the BD method.

The present invention has been made in view of the above-mentionedproblems, and therefore has an object to obtain a transmission device, acommunication system, and a precoding computation method, which arecapable of providing transmission precoding having a higher transmissiondiversity gain than that of the related-art BD method.

Solution to Problem

According to one embodiment of the present invention, there is provideda transmission device, which is to be applied to a communication systemin which a plurality of user terminals each including one or a pluralityof reception antennas exist for a transmission device including aplurality of transmission antennas so that multi-user MIMO downlinkcommunication is carried out, the transmission device including: aprecoder configured to apply transmission precoding processing to atransmission signal to be transmitted to each of the plurality of userterminals; a nonlinear processor configured to carry out successiveinterference cancellation for the transmission signal to which thetransmission precoding processing has been applied; and a beam formingcontroller configured to form a plurality of beams, a number of which issmaller than a number of the plurality of transmission antennas, basedon the transmission signal for which the successive interferencecancellation has been carried out, and to control a phase shifter or adigital signal processor, which is connected to the plurality oftransmission antennas, to control a beam radiation direction, whereinthe transmission device further includes a user ordering processorconfigured to estimate a positional relationship among the plurality ofuser terminals from radiation direction information on each of theplurality of beams held by the beam forming controller, order users inascending order of a distance from a specific user, determine a pairingof a transmission target user and an interference permissible user, andtransmit a determination result to the precoder, and wherein theprecoder is configured to determine two users of the transmission targetuser and the interference permissible user for each user based on thedetermination result received from the user ordering processor, andcarry out null steering for users other than the two users, to therebycarry out the transmission precoding processing.

Further, according to one embodiment of the present invention, there isprovided a communication system including: the plurality of transmissiondevices of the present invention; and a control station configured tosupervise and control the plurality of transmission devices, wherein thecontrol station is configured to generate beam direction information andordering information on each of the plurality of transmission devices sothat radiation regions of beams are inhibited from overlapping oneanother among the plurality of transmission devices, and then transmitthe beam direction information and the ordering information to each ofthe plurality of transmission devices, and wherein each of the pluralityof transmission devices is configured to control the beam radiationdirection based on the beam direction information, and carry out thetransmission precoding processing based on the ordering information.

Further, according to one embodiment of the present invention, there isprovided a precoding computation method, which is carried out in atransmission device to be applied to a communication system in which aplurality of user terminals each including one or a plurality ofreception antennas exist for a transmission device including a pluralityof transmission antennas so that multi-user MIMO downlink communicationis carried out, the precoding computation method including: a first stepof applying transmission precoding processing to a transmission signalto be transmitted to each of the plurality of user terminals; a secondstep of carrying out successive interference cancellation for thetransmission signal to which the transmission precoding processing hasbeen applied; and a third step of forming a plurality of beams, a numberof which is smaller than a number of the plurality of transmissionantennas, based on the transmission signal for which the successiveinterference cancellation has been carried out, and controlling a phaseshifter or a digital signal processor, which is connected to theplurality of transmission antennas, to control a beam radiationdirection, wherein the precoding computation method further includes afourth step of estimating a positional relationship among the pluralityof user terminals based on the beam radiation directions controlled inthe third step, ordering users in ascending order of a distance from aspecific user, and determining a pairing of a transmission target userand an interference permissible user, and wherein the first stepincludes determining two users of the transmission target user and theinterference permissible user for each user based on the pairingdetermined in the fourth step, and carrying out null steering for usersother than the two users, to thereby carry out the transmissionprecoding processing.

Advantageous Effects of Invention

According to the embodiments of the present invention, there is providedthe configuration capable of carrying out the precoding that permits theuser next to the target user as the IUI user, to thereby hierarchize themulti-user space in the MU-MIMO downlink. As a result, the transmissiondevice, the communication system, and the precoding computation methodcapable of providing the transmission precoding having a highertransmission diversity gain than that of the related-art BD method canbe obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram for illustrating an MU-MIMO downlink,which is a subject of the present invention.

FIG. 2 is a diagram for illustrating an example of a hybridconfiguration of an array antenna in the present invention, which is afull-array type hybrid configuration.

FIG. 3 is a diagram for illustrating an example of the hybridconfiguration of the array antenna in the present invention, which is asub-array type hybrid configuration.

FIG. 4 is a diagram for illustrating a full-digital type configurationof the array antenna in the present invention.

FIG. 5 is an explanatory diagram for illustrating an array antennaconfiguration in the present invention, in which beam forming units areunified.

FIG. 6 is a configuration diagram for illustrating a communicationsystem of a first embodiment of the present invention.

FIG. 7 is a diagram for illustrating a configuration of a transmissiondevice of the first embodiment of the present invention.

FIG. 8 is a diagram for illustrating a configuration of a receptiondevice in the first embodiment of the present invention.

FIG. 9 is a configuration diagram for illustrating a mobilecommunication system of a second embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

A description is now given of a transmission device, a communicationsystem, and a precoding computation method according to preferredembodiments of the present invention with reference to the drawings. Aphysical transmission antenna or reception antenna is hereinafterreferred to as “antenna”. Further, an effective array element observedwhen a transmission array or a reception array is multiplied by a weightis hereinafter referred to as “branch”.

In first and second embodiments of the present invention, for the sakeof simple description, it is assumed that the number of receptionantennas of each user terminal is N_(r) independently of the userterminal. Moreover, it is assumed that reception signals are multipliedby N_(w) (N_(w)≥N_(r)) weights in the reception array, to thereby setthe number of reception branches per user observed from the transmissionstation to N_(w) independently of the user terminal. As a result, atotal number of branches of all users is given as follows.N _(w,total) =N _(usr) ×N _(w)

In this case, the weights to be applied to the reception array areassumed in calculation of a precoding matrix, and any weights can beapplied. Moreover, when N_(w)=N_(r), the precoding matrix may be a unitmatrix or an eigenvector matrix of a channel matrix, and is not limitedin the present invention.

The first and second embodiments described below are applicable to anynumber of reception branches. Thus, as illustrated in FIG. 1, the numberof antennas and the number of branches may vary for each the user.

Moreover, the number of transmission antennas and the number ofreception branches are assumed to satisfy the following relationship.N _(t) ≥N _(w,total) −N _(w)=(N _(usr)−1)×N _(w)

A plurality of configuration methods for the array antenna areconceivable. In the Massive MIMO having a very large number of antennasand the like, a hybrid configuration in which the analog beam formingand the MIMO signal processing are combined with each other is used.FIG. 2 is a diagram for illustrating an example of the hybridconfiguration of the array antenna in the present invention, which is afull array type hybrid configuration. In the configuration of FIG. 2,N_(b), which is the number of beams, sets of phase shifters eachconnected to the antenna are prepared.

Each of the N_(b) DACs outputs a signal transmitted from each of thebeams. The transmission beams are formed by all the antennas by settingthe phase shifters. In general, the number of beams is sufficientlysmaller than the number of antennas. Therefore, the required numbers ofdigital devices, namely, the numbers of the DACs and ADCs aresuppressed, and thus the structure can be simplified.

FIG. 3 is a diagram for illustrating an example of the hybridconfiguration of the array antenna in the present invention, which is asub-array type hybrid configuration. In the configuration of FIG. 3, theantennas are divided into sub-arrays, and a beam is formed for each ofthe sub-arrays by phase shifters.

Compared with the full-array configuration, in the sub-arrayconfiguration, the number of antennas available for one beam decreases,but as many phase shifters as the number of antennas are only requiredto be prepared. Therefore, an analog portion is simplified. However, inthe sub array configuration, as many digital devices as the number ofbeams arc required as in the full-array configuration.

The configurations of FIG. 2 and FIG. 3 are often used in the case ofthe analog-digital hybrid, but may also be used in the digitalMassive-MIMO. FIG. 4 is a diagram for illustrating a full-digital typeconfiguration of the array antenna in the present invention. In theconfiguration of FIG. 4, as many digital devices as the number ofantennas are required.

Processing equivalent to the beam forming carried out by the phaseshifters is carried out through digital signal processing by a beamforming unit (“Beam Forming”) in the full digital configuration.

This configuration cannot reduce the number of digital devices, but thecomputation in MIMO Processing can be carried out through processing atas many spatial degrees of freedom not as the number N_(t) of antennas,but as the number N_(b) of beams. Therefore, a computation processingamount can be reduced. The beam forming unit can be adapted to both ofthe processing for the full-array and the processing for the sub-array.

The present invention can be applied to any one of the configurations ofthe array antenna and the beam forming methods of FIG. 2 to FIG. 4.Thus, a description is given of a configuration of FIG. 5, in which thebeam forming units are unified. FIG. 5 is an explanatory diagram forillustrating the array antenna configuration in the present invention,in which the beam forming unit is unified.

In the first embodiment, downlink communication from a base station touser terminals is assumed to be carried out, the base station is assumedto be a transmission device, and a user terminal is assumed to be areception device. As a type of the secondary modulation method, themulti-carrier type such as the orthogonal frequency division multiplex(OFDM) or the like may be applied, or the single-carrier type such asthe single-carrier block transmission or the like may be applied.

In the following description, a method that is represented by the OFDMand the single-carrier block transmission, which divides a signal intoblocks through the discrete Fourier transform processing and addition ofthe cyclic prefix (CP), is referred to as “block transmission”. Thepresent invention is not limited by the first and second embodimentsdescribed later.

Before the specific embodiments are described, the MU-MIMO downlinksystem configured to carry out the precoding is modeled by numericalexpressions. For a user # i (i=1, 2, . . . , N_(usr)), a transmissionsignal vector is indicated by s_(i)(t), a transmission powerdistribution matrix is indicated by P_(i), a transmission precodingmatrix is indicated by B_(i), a weight for forming a transmission beamis indicated by F_(i), and a true N_(r)×N_(t) channel matrix is giver,as follows:

-   -   Ĥ_(i)

An N_(w)×N_(r) reception weight matrix is indicated by W_(i), a truereception signal vector before reception weight multiplication isindicated by y_(i)(t), a reception signal vector after the receptionweight multiplication is indicated by r_(i)(t), and a true receptionthermal noise vector is given as follows:

-   {circumflex over (n)}_(i) (t)    Then, a system model can be defined by the expression below.

$\begin{bmatrix}{r_{1}(t)} \\\vdots \\{r_{N_{usr}}(t)}\end{bmatrix} = {{\begin{bmatrix}W_{1} & \ldots & 0 \\\vdots & \ddots & \vdots \\0 & \ldots & W_{N_{usr}}\end{bmatrix}\begin{bmatrix}{y_{1}(t)} \\\vdots \\{y_{N_{usr}}(t)}\end{bmatrix}} = \begin{bmatrix}W_{1} & \ldots & 0 \\\vdots & \ddots & \vdots \\0 & \ldots & W_{N_{usr}}\end{bmatrix}}$ $\left( {{{{{\left\lbrack \begin{matrix}{\hat{H}}_{1} \\\vdots \\{\hat{H}}_{N_{usr}}\end{matrix} \right\rbrack\left\lbrack \begin{matrix}F_{1} & \ldots & F_{N_{b}}\end{matrix} \right\rbrack}\left\lbrack \begin{matrix}B_{1} & \ldots & B_{N_{usr}}\end{matrix} \right\rbrack}\left\lbrack \begin{matrix}\sqrt{P_{1}} & \ldots & 0 \\\vdots & \ddots & \vdots \\0 & \ldots & \sqrt{P_{N_{usr}}}\end{matrix} \right\rbrack}\mspace{545mu}\left\lbrack \begin{matrix}{s_{1}(t)} \\\vdots \\{s_{N_{usr}}(t)}\end{matrix} \right\rbrack} + \begin{bmatrix}{{\hat{n}}_{1}(t)} \\\vdots \\{{\hat{n}}_{N_{usr}}(t)}\end{bmatrix}} \right)$

A digital MIMO processing unit of FIG. 5 cannot observe a channel ofeach of the antennas, and observes a channel for each beam formed in aspecific direction.

An N_(w)×N_(b) matrix obtained by multiplying the reception weightmatrix, the true channel matrix, and the transmission beam formationweights F by one another is set as a new channel matrix H_(i). AnN_(w)th-order vector obtained by multiplying the true reception thermalnoise vector by the reception weight matrix is set as a new receptionthermal noise vector n_(i). In this case, the system model is given bythe expression described below.

$\begin{matrix}{\begin{bmatrix}{r_{1}(t)} \\\vdots \\{r_{N_{usr}}(t)}\end{bmatrix} = {{\begin{bmatrix}H_{1} \\\vdots \\H_{N_{usr}}\end{bmatrix}\begin{bmatrix}B_{1} & \ldots & B_{N_{usr}}\end{bmatrix}}{\quad{{\begin{bmatrix}\sqrt{P_{1}} & \ldots & 0 \\\vdots & \ddots & \vdots \\0 & \ldots & \sqrt{P_{N_{usr}}}\end{bmatrix}\begin{bmatrix}{s_{1}(t)} \\\vdots \\{s_{N_{usr}}(t)}\end{bmatrix}} + \begin{bmatrix}{n_{1}(t)} \\\vdots \\{n_{N_{usr}}(t)}\end{bmatrix}}}}} & (1)\end{matrix}$

Expression (1) can be expressed by Expression (2).r (t)= H B P s (t)+ n (t)  (2)

In this case, in Expression (2),

-   H    is defined as an N_(w,total)×N_(b) system channel matrix.-   B    is defined as an N_(b)×N_(st) system precoding matrix (N_(st) is a    total number of streams to all the users).-   P    is defined as a system transmission power matrix, which defines the    transmission power distribution to the users.-   s(t)    is defined as an N_(st)th-order system transmission vector.-   n(t)    is defined as an N_(w,total)-th-order system noise vector after    multiplication of the reception weight.

A product of

-   H    and-   B    can be considered as an effective system channel matrix-   H _(e)    through transmission branch formation, and is given by Expression    (3).

$\begin{matrix}{{\overset{\_}{H}}_{e} = \begin{bmatrix}{H_{1}B_{1}} & {H_{1}B_{2}} & \ldots & {H_{1}B_{N_{usr}}} \\{H_{2}B_{1}} & {H_{2}B_{2}} & \ldots & {H_{2}N_{N_{usr}}} \\\vdots & \vdots & \ddots & \vdots \\{H_{N_{usr}}B_{1}} & {H_{N_{usr}}B_{2}} & \ldots & {H_{N_{usr}}B_{N_{usr}}}\end{bmatrix}} & (3)\end{matrix}$

A transmission precoding method involving, in the effective systemchannel matrix given by Expression (3), only keeping block diagonalelements, namely, H_(i)B_(i) (i=1, 2, . . . , N_(usr)), and setting azero matrix O to each of the other non-block diagonal elements is arelated-art BD method disclosed in Non Patent Literatures 1 and 2.Moreover, an effective system channel matrix given by Expression (4) isthe block bi-diagonalization (BBD) disclosed in Non Patent Literature 3.

$\begin{matrix}{{\overset{\_}{H}}_{e} = \begin{pmatrix}{H_{1}B_{1}} & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\{H_{2}B_{1}} & {H_{2}B_{2}} & 0 & 0 & 0 & 0 & 0 & 0 \\0 & {H_{3}B_{2}} & {H_{3}B_{3}} & 0 & 0 & 0 & 0 & 0 \\0 & 0 & {H_{4}B_{3}} & {H_{4}B_{4}} & 0 & 0 & 0 & 0 \\0 & 0 & 0 & {H_{5}B_{4}} & {H_{5}B_{5}} & 0 & 0 & 0 \\0 & 0 & 0 & 0 & {H_{6}B_{5}} & {H_{6}B_{6}} & 0 & 0 \\0 & 0 & 0 & 0 & 0 & {H_{7}B_{6}} & {H_{7}B_{7}} & 0 \\0 & 0 & 0 & 0 & 0 & 0 & {H_{8}B_{7}} & {H_{8}B_{8}}\end{pmatrix}} & (4)\end{matrix}$

The BBD method is a BMD method in which the number of IUI permissibleusers is set to one. Expression (4) corresponds to an example in whichN_(usr) is eight. It can be understood that a first user obtains thesecond diversity order by permitting interference to a second user. Fora user pair having residual IUI, interference is canceled through IUI-PCwhen the transmission signal is generated.

In the following, a detailed description is now given of the firstembodiment and the second embodiment based on those technical detailsdescribed above.

First Embodiment

FIG. 6 is a configuration diagram for illustrating a communicationsystem of the first embodiment of the present invention. Thecommunication system of the first embodiment includes a base stationincluding N_(t) transmission antennas 101, namely, a transmission device100, and a plurality of user terminals each including N_(r) receptionantennas 201, namely, N_(usr) reception devices 200(1), . . . , 200(N_(usr)).

Configurations and signal processing of the respective reception devices200(1) to 200(N_(usr)) are the same as one another. Thus, a descriptionof the reception device given below focuses on one certain receptiondevice, and the description is given while this reception device isassumed as the reception device 200.

FIG. 7 is a diagram for illustrating a configuration of the transmissiondevice 100 of the first embodiment of the present invention. Thetransmission device 100 includes primary modulators 110(1) to 110(N_(usr)), a precoder 120, a user ordering processor 121, a non-linearprocessor 130, a beam forming processor 140, and a beam formingcontroller 141.

Each of the N_(usr) primary modulators 110 applies primary modulation todata for each of the users. The precoder 120 carries out the MU-MIMOprecoding and the IUI-PC. During the MU-MIMO precoding and the IUI-PC,the user ordering processor 121 instructs the precoder 120 to carry outordering of the users and power distribution in precoding application.

The non-linear processor 130 applies non-linear processing to data afterthe processing by the precoder 120. The beam forming processor 140 usesthe plurality of antennas 101 to form beams. In this case, the beamforming controller 141 instructs a radiation direction of each of thebeams.

As other configurations, transmission waveform shapers each configuredto carry out secondary modulation, digital/analog (D/A) conversion,conversion from a baseband to a radio frequency (RF), and the like existas function units in the transmission device 100. As illustrated in FIG.2 to FIG. 5, the various configuration methods are conceivable, and itis thus assumed that the transmission waveform shapers are included inthe beam forming processor 140.

In this case, the beam forming processor 140 is assumed to carry outmapping to primary modulation symbols, such as channel coding orquadrature amplitude modulation (QAM), in the primary modulation.Moreover, the beam forming processor 140 is assumed to include afunction unit relating to discrete Fourier transform processing when thesingle carrier block transmission method is used.

Moreover, the transmission waveform shapers in the beam formingprocessor 140 are assumed to carry out the discrete Fourier transformand the CP addition processing before the D/A conversion when the blocktransmission is used. Moreover, signal processing means in thetransmission waveform shapers may be of a digital processing type or ananalog processing type.

When an input signal from the primary modulator 110 to the precoder 120is indicated as s110, s110 corresponds to

-   s(t)    of Expression (2).    When an output signal from the non-linear processor 130 to the beam    forming processor 140 is indicated as s130, s130 corresponds to a    portion of Expression (2) given as follows:-   B P s(t)

FIG. 8 is a diagram for illustrating a configuration of the receptiondevice 200 in the first embodiment of the present invention. Thereception device 200 includes reception waveform shapers 210(1) to210(N_(r)), a decoder 220, and a demodulator 230.

Each of the N_(r) reception waveform shapers 210(1) to 210 (N_(r))carries out conversion from the RF to the baseband, analog/digital (A/D)conversion, signal filtering processing, and the like for a receptionsignal from each of the reception antennas 201. The decoder 220 carriesout MU-MIMO decoding processing to extract a desired signal. Further,the demodulator 230 applies de-mapping and channel decoding processingto the signal after the MU-MIMO decoding.

In this case, the reception waveform shaper 210 is assumed to carry outCP removal and discrete Fourier transform before input to the decoder220 when the block transmission is used. Then, signal processing meansin the reception waveform shaper 210 may be of a digital processing typeor an analog processing type.

Moreover, it is assumed that the demodulator 230 includes equalizationprocessing of compensating frequency distortion and discrete inverseFourier transform processing when the single carrier block transmissionis used.

A description is now given of a calculation method for an MU-MIMOprecoding matrix to be applied in the precoder 120 in the firstembodiment. The present invention is applicable to any IUI permissibleuser in the BMD method. However, in the first embodiment, a descriptionis given of an example (BBD method) in which the number of IUIpermissible users is one.

In the following description, for the sake of simple discussion, theprocessing by the transmission waveform shapers included in the beamforming processor 140 and the reception waveform shapers 210 is omitted,and it is assumed that a portion between the output of the precoder 120of the transmission device 100 and input to the decoder 220 of thereception device 200 is expressed as an equivalent baseband system.

Moreover, the following calculation process requires information on thechannel matrix in the precoder 120. This information on the channelmatrix can be provided by feedback of channel information estimated bythe reception device 200 to the transmission device 100 when thedownlink and the uplink are constructed based on frequency divisionduplex (FDD). Moreover, when the downlink and the uplink are constructedbased on time division duplex (TDD), the channel information can beacquired through estimation of the channel by the transmission device100 based on reversibility of the transmission/reception.

In order to obtain the precoding matrix for the user # i, one user theIUI for whom is permitted is selected in addition to the desired user #i. In the following description, the one user to be selected is referredto as “IUI user”. The user once selected as an IUI user cannot beselected as an IUI user for the calculation of the precoding matrix foranother user # p (p≠i).

In other words, the same user cannot be selected a plurality of times asan IUI user. As a supplementary description, when the number of IUIpermissible users is M, the same user can be selected as an IUI user upto M times.

As an example, a description is given of a specific computation methodfor the precoding matrix when a user (# i+1) having the index next tothat of the user # i is selected as an IUI user. From a system channelmatrix given below,

-   H    an (N_(w,total)−2N_(w))×N_(b) matrix, which is given by-   H _(i)    and from which elements of the transmission matrix corresponding to    the desired user # i and the IUI user # i+1 are removed, is defined.    It is assumed that singular value decomposition (SVD) can be carried    out as given by Expression (5).

$\begin{matrix}{{\overset{\_}{H}}_{i} = {\begin{bmatrix}H_{l} \\\vdots \\H_{i - 1} \\H_{i + 2} \\\vdots \\H_{N_{usr} - 1}\end{bmatrix} = {{U_{i}{\sum_{i}V_{i}^{H}}} = {{U_{i}\begin{bmatrix}\sum_{i}^{(s)} & 0 \\0 & 0\end{bmatrix}}\begin{bmatrix}V_{i}^{{(s)}H} \\V_{i}^{{(n)}H}\end{bmatrix}}}}} & (5)\end{matrix}$

In Expression (5), U_(i) is a left singular vector matrix of,

-   H _(i),    V_(i) is a right singular vector matrix thereof, and Σ_(i) is a    singular value matrix having singular values as diagonal elements.

In the singular value matrix Σ_(i), when singular values of the diagonalelements are arranged in descending order of the magnitude, as given byExpression (5), the singular value matrix ΣE_(i) can be divided into,and can be expressed as a partial diagonal matrix Σ_(i) ^((s))constructed of (N_(w,total)−2N_(w)) non-zero singular values and zeromatrices corresponding to (N_(t)−(N_(w,total)−2N_(w))) zero singularvalues.

Moreover, right singular vectors V_(i) ^((s)) and V_(i) ^((n)) exist soas to correspond to the respective matrices expressed in the dividedform. When V_(i) ^((n)) is defined as a precoding matrix for the user #i, an effective channel matrix for the user # i is given by Expression(6), and the null steering is applied to the users other than the user #i and the user # i+1.

$\begin{matrix}{{\overset{\_}{H}}_{e,i} = {{\overset{\_}{H}V_{i}^{(n)}} = \begin{bmatrix}\vdots \\0 \\{H_{i}V_{i}^{(n)}} \\{H_{i + 1}V_{i}^{(n)}} \\0 \\\vdots\end{bmatrix}}} & (6)\end{matrix}$

Further, a beam space appropriate for the user # i is formed from thedesired element H_(i)V_(i) ^((n)) for the user # i in the Expression(6). In other words, the SVD is applied to H_(i)V_(i) ^((n)), or theeigenvalue decomposition is applied to non-negative value Hermitianmatrix, (H_(i)V_(i) ^((n)))^(H)H_(i)V_(i) ^((n)), to thereby obtain aneigenvector matrix V_(i) ^((n)) corresponding to large eigenvalues.

After a signal space is formed only for the user # i and the user # i+1by multiplying V_(i) ^((e)) after the null steering through V_(i)^((n)), beam forming for increasing a reception gain at the user # i canbe achieved. As a result, the precoding matrix for the user # i in thepresent invention is given by Expression (7).B _(i) =V _(i) ^((n)) V _(i) ^((e))  (7)

An IUI user cannot be set for the last user. Therefore, a precodingmatrix that does not permit interference and thus achieves a completenull steering for the other users is acquired as in the ordinary BD.Specifically, an (N_(w, total)=N_(w))×N_(b) matrix that is obtained byremoving desired channel elements from the system channel matrix and isgiven as follows is defined for the user # N_(usr).

-   H _(N) _(usr)    It is assumed that singular value decomposition given by    Expression (8) can be carried out.

$\begin{matrix}{{\overset{\_}{H}}_{N_{usr}} = {\begin{bmatrix}H_{1} \\H_{2} \\\vdots \\H_{N_{usr} - 1}\end{bmatrix} = {{U_{N_{usr}}{\sum_{N_{usr}}V_{N_{usr}}^{H}}} = {{U_{N_{usr}}\begin{bmatrix}\sum_{N_{usr}}^{(s)} & 0 \\0 & 0\end{bmatrix}}\begin{bmatrix}\sum_{N_{usr}}^{{(s)}H} \\V_{N_{usr}}^{{(n)}H}\end{bmatrix}}}}} & (8)\end{matrix}$

In Expression (8), U_(Nusr) is a left singular vector matrix

-   H _(N) _(usr)    V_(Nusr) is a right singular vector matrix thereof, and Σ_(Nusr) is    a singular value matrix having singular values as diagonal elements.    In the singular value matrix Σ_(Nusr), when singular values of the    diagonal elements are arranged in descending order of the magnitude,    as given by Expression (8), the singular value matrix Σ_(Nusr) can    be divided into, and can be expressed as a partial diagonal matrix    Σ_(Nusr) ^((s)) constructed of (N_(w, total)−N_(w)) non-zero    singular values and zero matrices corresponding to    (N_(b)−(N_(w,total)−N_(w))) zero singular values.

Moreover, right singular vectors V_(Nusr) ^((s)) and V_(Nusr) ^((n))corresponding to the respective matrices exist. When V_(Nusr) ^((n)) isdefined as a precoding matrix for the user # N_(usr), an effectivechannel matrix for the user # Nusr is given by Expression (9), and thenull steering is applied to the users other than the user # N_(usr).

$\begin{matrix}{{\overset{\_}{H}}_{e,N_{usr}} = {{\overset{\_}{H}V_{N_{usr}}^{(n)}} = {\begin{bmatrix}{H_{1}V_{N_{usr}}^{(n)}} \\{H_{2}V_{N_{usr}}^{(n)}} \\\vdots \\{H_{N_{usr} - 1}V_{N_{usr}}^{(n)}} \\{H_{N_{usr}}\; V_{usr}^{(n)}}\end{bmatrix} = \begin{bmatrix}0 \\0 \\\vdots \\0 \\{H_{N_{usr}}V_{N_{usr}}^{(n)}}\end{bmatrix}}}} & (9)\end{matrix}$

As described above, when the precoding matrices that permit theinterference to the next user for the user #1 to the user #(Nuser−1) andachieve the null steering for the other users are obtained, and areapplied to the system channels, an effective system channel matrix givenby Expression (10) is observed.

$\begin{matrix}{{\overset{\_}{H}}_{e} = \begin{bmatrix}{H_{1}B_{1}} & 0 & 0 & \ldots & 0 & 0 \\{H_{2}B_{1}} & {H_{2}B_{2}} & 0 & \ldots & 0 & 0 \\0 & {H_{3}B_{2}} & {H_{3}B_{3}} & \ddots & \vdots & \vdots \\\vdots & \vdots & \ddots & \ddots & \ddots & \vdots \\0 & 0 & \ldots & {H_{N_{usr} - 1}B_{N_{usr} - 2}} & {H_{N_{usr} - 1}B_{N_{usr} - 1}} & 0 \\0 & 0 & \ldots & 0 & {H_{N_{usr}}B_{N_{usr} - 1}} & {H_{N_{usr}}B_{N_{usr}}}\end{bmatrix}} & (10)\end{matrix}$

As appreciated from Expression (10), the effective system channel matrixis block bi-diagonalized. In other words, hierarchization, in which thenumber of interference permissible users is one, is achieved. As aresult, non-linear MU-MIMO processing that carries out successiveinterference cancellation on the transmission side can be applied asdescribed below.

The non-linear processor 130 utilizes the above-mentioned blockbi-diagonalization, to thereby be able to remove elements, which becomethe IUI on the reception side, on the transmission side in advance. Fromthe effective system channel matrix given by Expression (10), thereception signal observed by a user # i (i≥2) is given by Expression(11).r _(i)(t)=H _(i) B _(i)√{square root over (P _(i))}s _(i)(t)+H _(i) B_(i−1)√{square root over (P_(i−1))}s _(i−i)(t)+n _(i)(t)  (11)

When a transmission signal s_(i−1)(t) to the user #(i−1) is determined,interference at a reception point can be canceled by setting S_(i)(t) toa signal given by Expression (12).

$\begin{matrix}{{{\overset{\sim}{s}}_{i}(t)} = {{s_{i}(t)} - {\frac{H_{i}B_{i - 1}\sqrt{P_{i - 1}}}{H_{i\;}B_{i}\sqrt{P_{i}}}{s_{i - 1}(t)}}}} & (12)\end{matrix}$

The IUI does not exist for the reception signal of the user #1.Therefore, the above-mentioned processing is not required to be applied.The IUI at each of the reception user terminals can be canceled inadvance by using the above-mentioned processing to successivelydetermine the transmission signal starting from the user #2.

Moreover, the number of interference cancellations can be limited tothat for one user by applying the precoding matrix in the firstembodiment, which is different from the general non-liner MU-MIMO, inwhich the number of transmission interference cancellations increases inproportion to the number of users. Therefore, a computation amount canbe reduced, and reduction in SNR can also be suppressed.

However, a signal actually transmitted through the transmissionprocessing given by Expression (12) presents an expansion or acontraction, and thus becomes unstable. Thus, the non-linear processor130 may apply processing of stabilizing the transmission signal waveformthrough a modulo operation disclosed in Patent Literature 4 orperturbation processing disclosed in Patent Literature 5.

As described above, the first embodiment has the configuration ofcarrying out the precoding that permits the user next to the target useras the IUI user in the MU-MIMO downlink. As a result, the non-linerMU-MIMO method that can hierarchize the multi-user space, can reduce thecomputation amount, and can suppress reduction in SNR can be achieved.

An arrangement order of the users is extremely important in a procedureof determining the interfering user of the above-mentioned precodingmatric computation. The user ordering processor 121 illustrated in FIG.7 determines the user order and the power distribution. The userordering processor 121 has functions to determine the user order, andinstructs the precoder 120 on the order and the power distribution. Theorder of the users and the pairing with the IUI user who becomes aninterfering/interfered user are important in the ordering.

Due to a nature of the linear precoding, when close users having a highchannel correlation are paired with each other, an effect of improvingthe reception SNR increases. As an example of criteria for the ordering,the following criteria can be given.

(Criterion 1) Ordering is determined in accordance with a channel gain(square of the Frobenius norm of the channel matrix) of each of theusers.

(Criterion 2) Ordering is determined in accordance with the maximumeigenvalue of each of the users.

(Criterion 3) Ordering is determined so that the correlation of thechannel matrices of the users next to each other becomes high.

However, this computation carried out each time the precoding weightsare calculated poses a problem in terms of the computation amount. Thus,the user ordering processor 121 in the first embodiment uses informationon the beam radiation directions of the beam forming controller 141 tocarry out the ordering.

Branch forming on two stages is carried out in the MU-MIMO system of thebeam forming type illustrated in FIG. 2 to FIG. 5. In other words, thearray antenna is controlled to form beams toward the positions of theusers on a first stage. Then, beam synthesis (precoding) is carried outto cancel the interferences among the streams through the MIMO signalprocessing on a second stage.

In this case, the beams formed through the beam forming on the firststage are rough beams formed by the analog devices. Thus, a main lobe ofeach of the beams formed on the first stage simply directs to each ofthe users, and side lobes of the beam becomes interferences for otherusers.

Meanwhile, when a plurality of streams are simultaneously transmitted,those interferences are required to be canceled. The interferencecancellation among streams on the second stage is strict cancellation ofthe interferences among the streams.

Particularly in the configurations of FIG. 2 and FIG. 3, the first stageis achieved through the control by the analog phase shifters, and thesecond stage is achieved through the digital signal processing. Further,variations in the positions of the users is much slower than ascheduling speed. Therefore, an operation frequency of the beam formingprocessor 140 on the first stage is often lower than the operation ofthe precoder 120 on the second stage.

Based on this nature, the user ordering processor 121 uses informationon the beam radiation direction to carry out effective ordering at a lowcomputation amount. A description is now given of a specific procedure.

The user ordering processor 121 receives the information on the beamradiation direction from the beam forming controller 141, to therebyestimate a positional relationship among the users. Statistically, asthe positions of the users become closer to each other, a correlationbetween channels becomes higher. Thus, the user ordering processor 121carries out the ordering of the users in accordance with the followingprocedure.

(Step 1) Select any user as user #1. Set i to 1.

(Step 2) Select a user at a position closest to the user # i as user #i+1.

(Step 3) Finish the procedure when the ordering for all the users isfinished. When a user who has not been selected yet exists, set i toi+1, and return to (Step 2).

As another ordering method, use of received power of each of the usersis conceivable. A main beam of each of the beams is directed toward adirection in which the user exists, and signal power from the user isthus considered to be dominant in this direction. Thus, the userordering processor 121 carries out the ordering based on the receivedpower of each of the beams observed by the beam forming controller 141,to thereby be able to pair users existing close to one another.

As the ordering, an arrangement in ascending order or descending orderof the received power is conceivable. Received power intensityinformation on each of the beams or each of the users may be acquiredfrom another function block, for example, a higher layer.

For the power distribution among streams, for example, a method ofdetermining the distribution based on a channel gain of each of theusers in accordance with the water filling theorem or a method ofdistributing power so that reception qualities of all the users areequalized can be given. In the first embodiment, the method for thepower distribution is not particularly prescribed.

The first embodiment is described while the example of the BBD, in whichthe number of IUI permissible users is one, is used. However, the numberof IUI permissible users may be any number. In this case, a linerprecoder configured to form null for the users other than the desireduser and the IUI permissible users operates, and then, the IUI-PC isapplied so that the interference from all the IUI users is suppressed.

Further, the number of IUI permissible users may change in accordancewith each of the streams. In this case, the block triangulation (BT)method described in Non Patent Literature 3 can be adopted.

Second Embodiment

In order to build a large-scale mobile communication system, a pluralityof base stations are required to operate in cooperation with each other.Thus, in the second embodiment, a description is given of aconfiguration including a control station 300 configured to superviseand control a plurality of base stations in an environment in which aplurality of base stations exist.

FIG. 9 is a configuration diagram for illustrating a mobilecommunication system of the second embodiment of the present invention.In FIG. 9, a configuration in which N_(bs) base stations 100 in thefirst embodiment exist, and N_(bs) base stations 100(1) to 100 (N_(bs))are controlled by the control station 300 is illustrated.

In the configuration illustrated in FIG. 9, it is important to considerthat radiation regions of the beams or radiation regions of the branchesdo not overlap one another among the plurality of base stations 100.Thus, the control station 300 includes a beam forming controller 301, auser ordering processor 302, and a precoder calculator 303, and carriesout beam control and calculates precoding weights for each of the basestations 100.

A description is now given of a specific operation of the controlstation 300 in the second embodiment.

The beam forming controller 301 in the control station 300 issues aninstruction to the beam forming controller 141 of each of the basestations 100, to thereby control the directions of the beams to beradiated. The user ordering processor 302 in the control station 300receives position information on users from the beam forming controller301, and carries out ordering of the users in ascending order of thedistance for each of the base stations 100.

Further, the precoder calculator 303 in the control station 300calculates the precoding weights based on the user ordering information,and notifies each of the base stations 100 of the precoding weights.Each of the base stations 100(1) to 100(N_(bs)) forms branches, andtransmits signals through processing similar to that in the firstembodiment based on the notified precoding weight information and userordering information.

As described above, according to the second embodiment, even in thestate in which the plurality of base stations operate in cooperationwith each other, the user ordering is carried out by the controlstation, to thereby be able to improve the transmission performance ofthe MU-MIMO.

In the second embodiment, only the user ordering information may benotified from the control station 300 to the base station 100, and thecalculation of the precoding weights may be carried out by each of thebase stations 100.

The invention claimed is:
 1. A transmission device, which is to beapplied to a communication system in which a plurality of user terminalseach including one or a plurality of reception antennas exist for atransmission device including a plurality of transmission antennas sothat multi-user MIMO downlink communication is carried out, thetransmission device comprising: a precoder configured to applytransmission precoding processing to a transmission signal to betransmitted to each of the plurality of user terminals; a nonlinearprocessor configured to carry out successive interference cancellationfor the transmission signal to which the transmission precodingprocessing has been applied; and a beam forming controller configured toform a plurality of beams, a number of which is smaller than a number ofthe plurality of transmission antennas, based on the transmission signalfor which the successive interference cancellation has been carried out,and to control a phase shifter or a digital signal processor, which isconnected to the plurality of transmission antennas, to control a beamradiation direction, wherein the transmission device further comprises auser ordering processor configured to estimate a positional relationshipamong the plurality of user terminals from radiation directioninformation on each of the plurality of beams held by the beam formingcontroller, order users in ascending order of a distance from a specificuser, determine a pairing of a transmission target user and aninterference permissible user, and transmit a determination result tothe precoder, and wherein the precoder is configured to determine twousers of the transmission target user and the interference permissibleuser for each user based on the determination result received from theuser ordering processor, and carry out null steering for users otherthan the two users, to thereby carry out the transmission precodingprocessing.
 2. A transmission device according to claim 1, wherein theprecoder is configured to carry out the null steering for users otherthan the two users, and then carry out eigenbeam forming for thetransmission target user.
 3. A transmission device according to claim 1,wherein the user ordering processor is configured to carry out theordering by repeating the following processing including: (Step 1)selecting a user as a user #1, and setting i to 1; (Step 2) selecting auser at a position closest to the user # i from users who are yet to beselected, and setting the selected user as user # i+1; and (Step 3) whena user who is yet to be selected exists, setting i to i+1 and returningto (Step 2) until the ordering for all users is finished.
 4. Atransmission device according to claim 1, wherein the beam formingcontroller has a function of observing received power intensity of eachof the plurality of formed beams, and wherein the user orderingprocessor is configured to carry out the ordering in ascending order ordescending order of received power based on the received powerintensity.
 5. A transmission device according to claim 1, wherein theprecoder is configured to set a number of permissible users ofinter-user interference for each user as a value equal to or larger thanone, determine as many users as the number of permissible users as theinterference permissible users, and carry out the null steering forusers other than the transmission target user and the interferencepermissible users, to thereby carry out the transmission precodingprocessing.
 6. A communication system, comprising: the plurality oftransmission devices of claim 1; and a control station configured tosupervise and control the plurality of transmission devices, wherein thecontrol station is configured to generate beam direction information andordering information on each of the plurality of transmission devices sothat radiation regions of beams are inhibited from overlapping oneanother among the plurality of transmission devices, and then transmitthe beam direction information and the ordering information to each ofthe plurality of transmission devices, and wherein each of the pluralityof transmission devices is configured to control the beam radiationdirection based on the beam direction information, and carry out thetransmission precoding processing based on the ordering information. 7.A communication system according to claim 6, wherein the control stationis configured to further generate precoding weights based on theordering information, and transmit the precoding weights to therespective plurality of transmission devices, and wherein each of theplurality of transmission devices is configured to carry out thetransmission precoding processing based on the ordering information andthe precoding weights.
 8. A precoding computation method, which iscarried out in a transmission device to be applied to a communicationsystem in which a plurality of user terminals each including one or aplurality of reception antennas exist for a transmission deviceincluding a plurality of transmission antennas so that multi-user MIMOdownlink communication is carried out, the precoding computation methodcomprising: a first step of applying transmission precoding processingto a transmission signal to be transmitted to each of the plurality ofuser terminals; a second step of carrying out successive interferencecancellation for the transmission signal to which the transmissionprecoding processing has been applied; and a third step of forming aplurality of beams, a number of which is smaller than a number of theplurality of transmission antennas, based on the transmission signal forwhich the successive interference cancellation has been carried out, andcontrolling a phase shifter or a digital signal processor, which isconnected to the plurality of transmission antennas, to control a beamradiation direction, wherein the precoding computation method furthercomprises a fourth step of estimating a positional relationship amongthe plurality of user terminals based on the beam radiation directionscontrolled in the third step, ordering users in ascending order of adistance from a specific user, and determining a pairing of atransmission target user and an interference permissible user, andwherein the first step includes determining two users of thetransmission target user and the interference permissible user for eachuser based on the pairing determined in the fourth step, and carryingout null steering for users other than the two users, to thereby carryout the transmission precoding processing.